1. Field of the Invention
This invention relates to a current mode digital-to-analog (D/A) converter, and more particularly, to a current mode D/A converter having configurations that exhibit a wide range of analog output voltages with good linearity.
2. Description of the Related Art
A current mode D/A converter is one in which several constant-current sources or constant-current stages are connected in parallel to an output terminal or node through respective switches and the output terminal in turn is connected to a load resistor. The switches, which are really electronic ones, are turned on and off by the control of input digital codes so that constant-currents from current sources flowing to the load through the output terminal are selectively controlled to obtain an analog output (refer to, for example, Japanese Laid-Open Patent Publication No. 289319/1989). In such D/A converters, the analog output voltage is the drop across the load resistor
For example, fifteen identical current sources, each of which generates constant-current I, are prepared in the D/A converter if an input digital code is 4 bits long. Then a definite number of constant-current sources are actually connected to the output terminal, the number of which corresponds to the unique value of a input digital code ranging from 0000 to 1111 in natural binary representation. If a input digital code is 1010 for example, ten current sources are actually connected to the output node. Therefore, the total current flowing through a load resistor R1 is 10I and the voltage drop across it is 10IR1.
The remarkable feature of this current mode type of D/A converter is that it performs fast digital to analog conversion. The principal shortcoming is that each output voltage level as a fraction of the full-scale value is limited. More specifically, as disclosed in the above mentioned Japanese Laid-Open Patent Publication, a load resistor and a constant-current source and a switch in each leg of the D/A converter are serially connected between a positive supply line and ground line, and the analog output voltage is obtained as drop across the load resistor. Voltage drops also appear across constant-current sources and switches, and limit the analog output voltages to a narrow range. For example, if voltage drops across a constant-current source and a switch are respectively 2 volts and 1 volt and voltage between a positive supply line and a ground line is 5 volts, then the analog output dynamic range is limited at most within 2 volts. Accordingly, the requirements for using a supply voltage effectively and obtaining a wide analog output range cannot be met in a current mode D/A converter.
In the D/A converter disclosed in the above mentioned Japanese Laid-Open Patent Publication, at least one MOS transistor is used as a constant-current source. If an operating point on Id-Vd coordinates of the MOS transistor is shifted to the linear region from the saturation region where it was operated in order to reduce voltage drop across it, then the linearity of analog output voltages would be degraded. And such A/D converter would be impractical for use.